it can be shared easily with the ui3.
Just enable it in webserver tab under BI settings and u can access the live bi feed with google chrome with another machine as long as they are the same network / vlan.
You would also want to create a user, make them non-admin, set up username and password.
You can then assign a specific camera to a group and limit their viewing to only that group (needed if you have more than 1 camera on BI).
You can limit their bandwidth, login time, schedule their access time, allow PTZ functions, if they can see recorded clips or not and more.
All of this is covered in BI's built-in 'Help"...search for "users".
UI3 does not have push notifications. Push notifications can be done via the Blue Iris iOS and Android apps, but those apps are not free. I believe you can turn off push notifications for specific Blue Iris users.
To share your UI3 with someone else over the internet, you would need to forward a port, set up dynamic dns, and send them the address. There are a ton of guides for how to do this. I'm sure even the Blue Iris help file goes into some depth on the subject.
So you want to share your live bi cam feeds?? Hahaha that made me laugh.
If you are not going to use a VPN and are simply configuring your BI webserver to access the internet through your router, than the person that you want to have access to your BI can connect using TonyR's direction above and you don't need to be on the same lan/vlan to have access to BI.